While traditional inspection methods provide value to plant operators, many have begun condition based inspection programs aimed at bolstering their mechanical integrity and preventive maintenance programs. With this in mind, many plants have chosen to inspect their piping systems using Guided Wave UT (GUL) Inspection. The benefits of this inspection include:

• Large sections of piping can be inspected rapidly and safely often with no scaffolding.
• Minimal insulation removal for Corrosion Under Insulation (CUI) inspections.
• Localized damage can be pinpointed and characterized as to length and depth.
• GUL is an excellent tool for cased crossings and un-piggable pipe.
• Point of contact corrosion can be found without lifting the pipe thus avoiding potential leaks and protecting the environment.

Can examine large areas very quickly, and it does not require use of coupling liquids. In addition to finding cracks, eddy current can also be used to check metal hardness and conductivity in applications where those properties are of interest, and to measure thin layers of nonconductive coatings like paint on metal parts. At the same time, eddy current testing is limited to materials that conduct electricity and thus cannot be used on plastics. In some cases, eddy current and ultrasonic testing are used together as complementary techniques, with eddy current having an advantage for quick surface testing and ultrasonic having better depth penetration.

Ultrasonic phased array systems can potentially be employed in almost any test where conventional ultrasonic flaw detectors have traditionally been used. Weld inspection and crack detection are the most important applications, and these tests are done across a wide range of industries including aerospace, power generation, petrochemical, metal billet and tubular goods suppliers, pipeline construction and maintenance, structural metals, and general manufacturing.

Time of Flight Diffraction or TOFD is one of the most promising ultrasonic techniques for the examination of welds on pressure vessels in lieu of radiography; for pipe weld quality or crack detection and also weld root erosion.

TOFD is a computerized ultrasonic system able to scan, store, and evaluate indications in terms of height (through wall thickness), length and position, with a degree of accuracy and speed never achieved with other ultrasonic techniques. Time of Flight Diffraction (TOFD) is one of the methods used for flaw detection and sizing. Of all the ultrasonic crack sizing methods, TOFD is the most accurate methods. The inspection employs two longitudinal wave (L-wave) angle beam transducers arranged symmetrically opposite facing each other, straddling the weld or base material under test. One probe acts like a transmitter of ultrasonic energy while the other probe receives the ultrasound energy. The transducer, pulser, and amplifier characteristics are selected to generate as broad distribution of energy as possible over the material under test providing full weld coverage. A single-axis scan (that is, along the weld), with a position encoder records the position of the weld and enables the display of digital images in real time.

Background:

Acoustic Emission (AE) testing was developed for a wide range of applications including the assessment of structural integrity on pressure vessels and storage tanks. Acoustic emission is the elastic energy released by materials when they undergo deformation. In metals, emission can be caused by a number of mechanisms including stress corrosion cracking, weld cracking, etc. Its use on product storage equipment has been on tanks and spheres containing high-risk products such as ammonia, butane and propane. The method is written into the ASME Boiler and Pressure Vessel Code (Section V Article XII) and is also extensively used to test other pressurized equipment and atmospheric equipment.

Benefits include:

• AE is non-intrusive and can be performed on in-service equipment.

• External AE testing is more cost effective than internal inspections.

• AE assesses the equipment for structural problems, approximates where flaws are, and gives.

• Little/no disturbance to insulation.

• Wide temperature range (Cryogenic to High temperature).

• Recognized by several standards.

Inspection Applications:

• To inspect metallic pressure vessels, tanks and piping as spheres, bullets.

• To inspect composite (FRP, GRP) Pressure Vessels, Tanks and Piping Tank farm lines.

• To inspect aboveground storage tank floors.

• To inspect refrigerated ammonia tanks.

• To inspect metallic vessels under thermal fatigue, as reactors and coke drums. A measure of their severity.

Automated Ultrasonic Testing (AUT) covers a range of ultrasonic inspection techniques using powered, mechanical scanners to locate inherent defects within a given material. AUT is the term used to describe corrosion mapping inspections, pulse-echo weld inspection, Phased Array and Time of Flight Diffraction.

Aut is applied
• To obtain repeatable corrosion maps, with this we:
• Measure flaws in the base material and show their position, extent and depth.
• Detect pitting, general corrosion and laminar defects including inclusions, plate laminations and blistering.
• To assess pressure vessels and piping for blistering, Hydrogen Induced Cracking (HIC also referred to as stepwise cracking) and Stress Oriented Hydrogen Induced Cracking (SOHIC) damage; for these inspections we can use ‘Tri-element’ probes.
• To differentiate between laminations, blistering and stepwise cracking.
• To inspect as-fabricated welds.

Advantages
• High degree of repeatability
• Position and size data for every flaw can be compared for repeat scans of the same area to track flaw growth or corrosion rates both generally and for individual pits.
Limitations
In-service inspections, especially high temperature examinations, require extensive preparation and cooperation between pressure containing operating personnel and AUT personnel.

Automated UT Weld Inspection is a term with potentially multiple interpretations; it may refer to:

• Various forms (mechanized and manual) of computer encoded UT weld inspections such as.

• Phased Array (PA) and Time of Flight Tip Diffraction (TOFD).

• Mechanized UT weld inspections.

• Routine weld inspections such as Advanced Ultrasonic Back Scatter and Spectrum Analysis (AUBT/ABSA).

Tank Floor MFL scanning is a non-destructive examination method which uses a magnetic field to detect corrosion and pitting in carbon steel. A powerful magnet is scanned close to the surface to ‘saturate’ the steel with the magnetic field. The magnetic field “leaks” from the steel where there is corrosion and this is detected by the scanner.

Between the scanner bridge magnetic poles, a near-saturation magnetic flux is induced in the material examined. The scanner sensor detects flux leakage changes when the plate thickness changes.

This may indicate the presence of discontinuities, such as pitting and corrosion, on the process and/or soil side. The scanner is moved over the entire tank bottom surface to provide the required inspection coverage. Technicians interpret the scanner display to identify damaged areas and in some cases, estimate the amount of metal loss. Thickness losses detected by ultrasound are reported and mapped in a CAD rendering of the floor.
MFL Scans is Performed
• On storage tank bottoms for all industries: upstream, mid-stream, petrochemical, refineries.
• To identify soil and product side metal losses, pitting and general corrosion.
Advantages
• Fast method for inspecting large areas.
• Minimal set-up time.
• Yields reliable and economic qualitative tank floor assessments. High sensitivity: acceptable sensitivity can be obtained through up to 0.500” of combined steel and coating thickness.
• Requires access to only one side of the material.
Limitations
• Not a quantitative technique for identifying remaining wall thickness.
• Requires ultrasonic follow up where indications of wall loss are found.
• Cannot differentiate between soil side and product side indications.
• Poor surface conditions (scale, debris, roughness, and certain coatings) may limit the integrity of the inspection.
• Internal tank components close to the floor limit the access to particular areas.